Size and ligand effects on the electrochemical and spectroelectrochemical responses of CdSe nanocrystals

Abstract

The electrochemical properties of CdSe quantum dots with electrochemically inactive surface ligands (TOPO) have been investigated in comparison with the analogous nanocrystals containing electrochemically active oligoaniline ligands. The TOPO-capped nanocrystals have been studied in a wide size range (from 3 to 6.5 nm) with the goal to amplify the influence of the quantum confinement effect on the electrochemical response. The determined HOMO and LUMO levels have been found in good agreement with the ones obtained from photoluminescence studies and those predicted theoretically. Ligand exchange with aniline tetramer significantly influences the voltammetric peaks associated with the HOMO oxidation and the LUMO reduction of the quantum dots, which are shifted to higher and lower potentials, respectively. These shifts are interpreted in terms of the positive ligand charging which precedes the oxidation of the nanocrystals and the insulating nature of the ligand in the case of the nanocrystal reduction. The ligand-nanocrystal interactions have also been studied by UV-Vis-NIR and Raman spectroelectrochemistry in comparison with a specially prepared model compound which, apart from the anchoring function is identical to the grafted oligoaniline ligand. Both spectroelectrochemical techniques clearly indicate the same nature of the oxidation/reduction pathway for both the model compound and the grafted ligand. The influence of the grafting is manifested by a shift in the onset of the ligand oxidation as compared to the case of the "free" model compound. Since both components (ligands and nanocrystals) mutually influence their electrochemical and spectroelectrochemical properties, the newly developed system can be considered as a true molecular hybrid. Such hybrids are of interest because the potential zone of the ligand electroactivity is well separated from that of the nanocrystals and, as a result, the organic part can be electrochemically switched between the semiconducting and the conducting states with no change in the oxidation state of the nanocrystal. The newly developed system offers therefore the possibility of an electrical addressing of individual nanocrystals via the conducting ligands.

Investigating the Control by Quantum Confinement and Surface Ligand Coating of Photocatalytic Efficiency in Chalcopyrite Copper Indium Diselenide Nanocrystals

“…This critical information cannot be determined by simple optical absorption measurements. Since the initial report of determining size-dependent electrochemical band-gap of dispersed CdS NCs in DMF/electrolyte, electrochemical properties of dispersed CdSe, [53][54][55][56][57][58][59][60] CdTe, 53,61,62 CdS x Se 1-x , 63 and CdSe x Te 1-x , 64 NCs were also investigated, but to the best of our knowledge size-dependent electrochemical characterization of CuInSe 2 NCs has not been reported. However, in all cases appearance of multiple peaks in the CV were observed, which could be either presence of free, unbound surface passivating ligands and/or existence of surface trap states of NCs.…”

Entrapment in phospholipid vesicles quenches photoactivity of quantum dots

“…Type and quality of surface passivating ligands are important for the optical and electrochemical properties of QDs. 58 Surface passivation by oxygen (chemical or physical adsorption) eliminates such defects, causing an increase in radiative recombination. 59 However, a series of redox processes mediates further surface oxidation in air 60 or under UV exposure 45 leading to SeO 2 , CdO, CdSeO 3 , and CdSeO 4 formation and particle size reduction.…”

Quantum confinement induced shift in energy band edges and band gap of a spherical quantum dot

“…On the other hand, the experimental works on quantum confinement by Kucur et al, [55] Inamdar et al, [56] and Querner et al, [57] were performed through cyclic voltammetry method and Jasieniak et al [58] investigated the size dependency of valence and conduction band-edge energies using photoelectron spectroscopy in air (PESA).…”